There is known in the art a device for automatic adjustment of a roll gap in a mill stand, which comprises one-chamber hydraulic cylinders adapted for prestressing the mill stand and mounted under the bottom roll chocks.
The hydraulic cylinders are provided with an electrohydraulic system to control the pressure of a fluid being fed to said hydraulic cylinders from a high-pressure fluid source.
The aforesaid electrohydraulic system comprises electric load cells adapted to absorb a rolling force, load cells to register a mill stand prestressing force and servovalves fitted with electric circuit for their control.
The electric load cells for absorbing a rolling force are mounted under the stand housing screws and on top roll chocks.
The electric load cells for registering the mill stand prestressing force are arranged or mounted intermediate the top-housing separator and bottom roll chocks.
The servovalves together with their electric control circuit and high-pressure fluid sources are disposed or located outside the mill stand.
The afore-described device for automatic adjustment of a roll gap in a mill stand operates in the following manner.
During the rolling operation, the rolling force is varied and registered by the electric load cell.
The signal from this load cell is applied to the electrohydraulic system for controlling the fluid pressure in the hydraulic cylinders. As a result, the servovalve is operated to alter the fluid pressure in the hydraulic cylinders thereby altering the stand prestressing force. Therefore, by effecting the prescribed alteration of the stand prestressing force in accordance with the rolling force, the automatic adjustment of the roll gap within a preset range is assured.
The aforesaid prior-art device allows for substantially accurate adjustment of a roll gap in a mill stand.
It is to be understood, however, that the servovalves require a highly purified fluid (oil), otherwise, the servovalves become unstable in operation and the device looses its operating dependability.
Moreover, the servovalves are rather complex in construction, expensive to manufacture and difficult to operate. The valves, as well as their electric control circuit, require the attendance of highly qualified personnel.
There is also known a device for automatic adjustment of a roll gap in a mill stand, which comprises load cells for absorbing a rolling force which are not affected by the action of the two-chamber hydraulic cylinders for prestressing the mill stand. Each of the hydraulic cylinders is fitted with two rods, one of which rests upon the roll housing cross bar and the other one accommodates therein a hydraulic load cell. Resting against the latter in a chock formed with openings through which rods are extended, each rod having one end thrust up against the body of the two-chamber hydraulic cylinder and the other end thrust against the opposite chock. The two-chamber hydraulic cylinder communicates through one of its chambers with a constant-pressure fluid source and through its other chamber, directly with the load cell chamber and with a means for varying the amount of oil in said chambers.
The afore-described device operates in the following manner.
During the rolling operation, the load cells take up the rolling force with a resultant change of fluid pressure therein, which, in turn, causes a change of pressure in the chambers of the two-chamber hydraulic cylinders in communication with these load cells. As a result, the mill stand prestressing force will vary, while the fluid pressure in the chambers of the two-chamber hydraulic cylinders, communicating with the constant-pressure fluid source, will remain unchanged. The change in the mill stand prestressing force will cause a change in the mill stand deformation and, consequently, in a roll gap between the working rolls.
By selecting the device parameters, those for the two-chamber hydraulic cylinders and load cells, a definite relationship is obtained between the mill stand prestressing force and the rolling force, which allows for roll-gap automatic adjustment in the mill stand.
The setting of the described prior-art device is effected by an appliance for varying the amount of fluid in the inter-communicated chambers of the load cell and one of the chambers of the two-chamber hydraulic cylinder with the purpose of varying the rigidity of the load cell and that of the mill stand, respectively.
However, the device of the type described is incapable of maintaining a constant roll-gap profile between the working rolls during the rolling operation. The reason for this is that the decreasing gap between the chocks, results in a fluid overflow from the load cell chamber into the chamber of the two-chamber hydraulic cylinder in communication therewith. This overflow causes the displacement of the chocks together with the rolls, which makes impossible compensation for the deflection of the rolls.